A Hubble Space Telescope image shows the core of M2, a globular cluster that is about 35,000 light-years from Earth. The cluster contains perhaps 150,000 stars, which are packed into a spherical region of space only about 200 light-years across. From a planet inside the cluster, the sky would abound with so many brilliant stars that there would be no true "night." The cluster's stars are about 12 billion years old, which makes them some of the oldest stars in the entire galaxy. [NASA/STScI]

Most of the stars that are visible in the night sky are youngsters — most are no more than a few billion years old, while some are still in the millions of years. But many of the galaxy’s stars are as old as the Milky Way itself — more than 12 billion years.

A whole bunch of those old stars congregate in a globular cluster known as M2. It’s low in the west at nightfall, in the constellation Aquarius. It’s not quite bright enough to see with the eye alone, but binoculars show it as a small, fuzzy blob of light.

The cluster is about 35,000 light-years away, and it spans more than 200 light-years. It contains perhaps 150,000 stars, all of which are less massive than the Sun.

When it was born, M2 probably contained lots of big, bright stars as well. But such stars burn out in a hurry. All that’s left are the smaller stars, which consume their nuclear fuel much more slowly than the heavier ones.

In fact, astronomers determine the cluster’s age by looking at the masses of the stars that are in the final stages of life. Heavier stars have already expired, while lighter ones are still in the prime of life.

The stars that are just now expiring are roughly as massive as the Sun. Models of how stars evolve show that those stars are about 12-and-a-half billion years old — which tells us that that’s also the age of the entire cluster. That makes the stars of M2 some of the oldest in the galaxy.